With the increase of integration scale of integrated circuits, the feature sizes of patterns tend to shrink more and more and, recently, a high-precise pattern profile of less than about 1 .mu.m in line width has been demanded. For meeting this demand, various methods have been proposed.
For example, from the aspect of apparatus, at the following two points:
(1) use of a light source of shorter wavelengths; and
(2) use of lens having a large numerical aperture have been proposed for increasing the resolution from optical theory, and test machines are marketed. However, when a light source having shorter wavelengths is used, the percent transmission of a lens is reduced and, thus, a sufficient amount of the light is not obtained. Also, when a lens having a large numerical aperture is used, there occurs a problem that the resolution becomes worse on the wafer with topography because the depth of focus becomes shallow.
On the other hand, from the aspect of the resist, the development of a resist having high contrast (hereinafter referred to as "high-gamma resist") has been attempted. A high-gamma resist surely improves the resolution to some extent. Also, the high-gamma resist can be produced by controlling the molecular weight and dispersion degree of a polymer in the resist. However, in the case of using such a high-gamma resist, there is a restriction on the material to be used, and the resolution is not improved to a great extent.
Also, from the aspect of lithography, there are proposed the following methods:
(1) a method of using a multilayer resist; and
(2) a method of spin-coating a photosensitive resin on a resist film.
The method of using a multilayer resist can provide patterns having a high resolution of less than about 1 .mu.m, but there are yet problems as an industrial technique in the method because the process is complicated, the production of devices requires a too long period of time, and the yield for the production of devices is low.
In the method of spin-coating a photosensitive resin on a resist film, the resolution is improved as follows. That is, light directly after passing through a mask has high contrast, but the contrast of the light is reduced after the light passes through space and a lens. In this regard, according to the improved method, the reduced contrast of the light is increased again by passing through the photosensitive resin to thereby improve the resolution.
As a photobleachable material contained in the photosensitive resin, there are proposed nitron compounds as described, for example, in Japanese Patent Application (OPI) No. 104642/84 or diazonium salts as described, for example, in Japanese Patent Application (OPI) No. 238829/85 (the term "OPI" as used herein means an "unexamined published patent application").
The method of using a nitron compound may certainly improve the resolution, but since in the method an interlayer is required between a resist layer as the lower layer and a photosensitive layer as an upper layer, there are problems that the process becomes complicated and the yield for the production of semiconductors is not so improved.
In the method of using a diazonium salt, the improvement effect for resolution is not yet satisfactory, and the diazonium salt to be used has problems in terms of storage stability.
The inventors have investigated on the method of using a diazonium salt as a photobleachable agent in the aforesaid methods of forming a photosensitive layer which is bleached by light based on the considerations that a method capable of simultaneously developing a photosensitive layer with the developer of the lower layer can simplify the process, and a great improvement of resolution can be expected in this case.
As the result of investigations using a method reported as an evaluation method for the mechanism of positive photoresist. (F. H. Drill et al, Characterization of Positive Photoresist, IEEE Transactions on Electron Devices, Vol. ED-22, No. 7, July 1975 by IBM Corp., U.S.A.), the inventors have discovered that the resolution can be greatly improved by increasing an A value as defined below.
The A value is generally shown by the following equation: ##EQU1## T(0): Initial Transmittance T(.infin.): Final Transmittance
d: Film Thickness
For increasing the A value, it can be considered to reduce the film thickness (d), give a high transmittance to the photosensitive layer (upper layer) after bleaching (that is, the value of T(.infin.) is near 100% as close as possible), and also reduce the initial transmittance T(0) as low as possible. In order to reduce the value of T(0), the concentration of a diazonium salt as a photobleachable agent in the photosensitive layer may be increased, but if the concentration thereof is increased, crystals of the photobleachable agent deposit after coating, which results in rather reducing the resolution.
Also, a method of adding an additive such as a sulfonic acid derivative or a salt thereof for increasing the solubility as disclosed in Japanese Patent Application (OPI) No. 238829/85 cannot increase the effect owing to the limitation on the addition amount thereof because if the amount is large, the coating properties of the photosensitive layer are reduced.
Furthermore, there is also a problem that the diazonium salt as a photobleachable agent is poor in heat stability.
Diazonium salts capable of absorbing a light of longer wavelength are generally poorer in stability. However, as described above, the lithography using light having a short wavelength has a problem that the light transmittance is lowered. Furthermore, manufacturers for semiconductor integrated circuits have a desire of elongating the life of a light-exposure apparatus (mainly using a light of 436 nm at present) in order to avoid an increase in investment for equipment and, hence, an improvement in stability of diazonium salts is an important factor in this field of technique.
Thus, when the stability of a diazonium salt was investigated by adding thereto a stabilizer such as phosphoric acid, organophosphoric acid, citric acid, tartaric acid, etc., an improvement effect was confirmed to some extent and, hence, when a large amount of the aforesaid additive was added for further improving the stability of the salt, it was further confirmed that the coating properties were reduced to thereby lower the resolution.
For solving the above-described problems, the inventors have made various investigations on polymers used for photosensitive resins, and as the result thereof, it has been discovered that by using a polymer containing a benzenesulfonic group, the solubility of the diazonium salt in a solvent is increased, the deposition of crystals of the diazonium salt at spin-coating is prevented, and further the stability of the diazonium salt can be improved.
Also, it has been found that by utilizing a functional group of the polymer in a photosensitive resin as a counter anion of the diazonium salt, a large amount of the diazonium salt can be dissolved without reducing the coating properties and, at the same time, the stability of the diazonium salt can be improved. As the result thereof, fine patterns can be produced with high precision, and based on these discoveries, the present invention has been attained.